The present invention relates to an electrical brake and in particular an electromechanical brake for motor vehicles, the brake being implemented by way of an electric motor and being equipped with a holding brake function.
An electrical brake, and in particular an electromechanical brake for motor vehicles that is implemented by way of an electric motor and is equipped with a holding brake function, is known. In known electromechanical brakes, in which the electric motor applies the brake caliper optionally via a spindle and a linkage, this holding brake function is implemented by the fact that a separate holding brake is added onto the motor. This immobilizes the rotor and thus the spindle on which the electric motor acts. In direct-current motors such as those predominantly used today for electromechanical brakes, the holding brake is implemented by way of a frictional engagement brake that is controlled via an electromagnetic coupling and acts on the rotor shaft. If the motor torque is to be held, the coupling is closed and the rotor is immobilized by way of the friction of the brake linings.
One particular problem with the known electromechanical brakes is the fact that the electric motor becomes thermally overloaded if the motor""s rotor is to be immobilized in a position and a large current is flowing. Such is the case in particular if the brake is actuated for a long period, for example if the driver of a motor vehicle keeps his or her foot pressed on the brake. In order to prevent this risk of electrical and thermal overload, the aforesaid separate holding brake is added onto the motor. When the rotor of this motor is retained by the mechanical brake, the motor current can be switched off. Before the motor rotates further, the holding brake is released and the motor can again be energized. In the example mentioned above, in which the holding brake is realized by way of an electromagnetically actuated frictional engagement brake, the motor can be switched to zero current once the holding brake has been activated. When the motor once again needs to rotate, the frictional engagement is disengaged via the coupling and the rotor is released. As a result, a situation can occur in an overlap period such that the electric motor is energized while the retaining brake is still engaged. This results in increased wear on the brake linings. In the control system, this results in dynamic losses and undesirable dead times if this overlapping state is to be prevented by way of control technology.
The electrical brake according to the present invention has, in contrast, the advantage of making available thermal relief for the motor without an additional holding brake. There are no dead times for the control system during activation and release of the holding brake function. Furthermore, there is no overlap between motor energization and immobilization of the electric motor by the holding brake. Reliability is enhanced by the elimination of components, the mechanical drive design is simplified, and there is a reduction in actuator mass and a decrease in overall volume.
For this purpose, in the electrical brake according to the present invention an electric motor with salient stator poles and rotor poles is provided in principle to generate the holding brake function, and the electric motor is acted upon by a defined holding current in such a way that the poles of the motor thereby are brought into a locked position and held there, or are held in a locked position.
According to a particularly advantageous embodiment of the present invention, upon activation of the holding brake function and after a locked position has been reached, the current delivered to the motor for braking, i.e. the braking current or motor current, is thereafter very substantially decreased. This ensures in all cases that the motor is not thermally overloaded when the holding brake function is activated. The holding current may be substantially less than the regulated maximum current, in particular less than 10% thereof.
According to an advantageous embodiment of the brake according to the present invention, provision is made for a reluctance motor or an electronically commutated direct-current motor to be provided as the electric motor.
In a further advantageous embodiment of the present invention, provision is made for the electrical brake to act via a spindle and optionally a linkage on the brake caliper, the electric motor operating on the spindle.
In an advantageous method for operating an electrical brake that possesses a braking motor having a holding brake function, the object of the present invention is achieved by the fact that when the braking current is applied for a long period, the holding function is activated and the electric motor is acted upon with a defined holding current in such a way that the poles of the motor thereby are brought into a locked position and held there, or are held in a locked position.
In a further advantageous embodiment of this method, provision is made that when the holding brake function is activated and once a locked position has been reached, the current delivered to the motor for braking, i.e. the braking current or motor current, is very substantially decreased, in particular to the holding current.
The brake configured according to the present invention, and the method according to the present invention, are advantageously used in an arrangement in which the brake is used on those spindles, axles, or wheels on which no provision is made for a parking brake or handbrake to act. This is of particular importance in an operating situation in which the parking brake, generally known as the handbrake, is also implemented with the holding brake, since in such a case a separate holding brake cannot be dispensed with; however, the latter need to be provided only on those wheels on which the parking brake or handbrake acts, for example only on the rear wheels. The other wheels then need no mechanical holding brake as is the case in the existing art.